A prior application in the name of the same Applicant, which is still pending, describes catalysts and supports for catalysts having a specific geometric shape, for example a cylindrical shape with through holes, with a circular or multilobed cross-section, obtained by tableting powders by using an external lubricant that is applied to the surface of the forming chamber and on the plungers of the mold instead of being dispersed in the bulk of the powder to be tableted.
The application does not describe catalysts for the oxichlorination of ethylene into 1,2-dichlorethane.
The synthesis of 1,2-dichlorethane by oxichlorination of ethylene can be performed, as known, in a fluidized-bed reactor or in a fixed-bed reactor. In the first case more uniform distribution of the temperature in the reactor is obtained (by avoiding localized overheating), with the detriment of some difficulty in fluidization, caused by the tendency of the catalyst particles to stick. In the second case, management of reaction parameters is easier, but due to the low heat exchange coefficient among the catalyst granules and between said granules and the reaction gas, localized temperature increases, known as "hot spots", can occur. These localized temperature increases must be avoided for reasons linked to the selectivity and useful life of the catalyst.
A first attempt to solve the problem of heat exchange among the granules of catalyst for the oxichlorination of ethylene has resorted to ring-shaped granules or to circular and cylindrical granules having a given height-to-diameter ratio.
The problem of the heat exchange coefficient is not the only technical problem to be solved in the case of an efficient synthesis of 1,2-dichlorethane in a fixed-bed reactor.
In fact, the following characteristics are also requested to a granular catalyst used in the fixed-bed oxichlorination of ethylene:
low resistance to the gas flow (low load loss for an equal thickness of the catalyst bed); PA1 a high effective surface, that is to say, a high surface-to-volume ratio; and PA1 good mechanical strength, in order to prevent breakage of the catalytic particles and consequent packing of the bed.
The catalysts that are normally used in the fixed-bed oxidative chlorination process (which are shaped like spheres, solid cylinders, or rings of various sizes) do not solve these problems satisfactorily. Moreover, when using these known configurations, diffusion of the reaction gases inside the granules of catalyst and complementary diffusion of the products from the inside of the granules are often very limited. This means that since the oxichlorinatlon reaction occurs more easily and selectively on the outer surface of the granule in the heterogeneous system taken into consideration, oxichlorination catalysts having known shapes are not used efficiently. Therefore, a large amount of catalyst has to be used in order to achieve the desired conversion, and thus tubes of adequate height have to be used in the case of tube-bundle fixed beds. With oxichlorination catalysts having known shapes, this entails a further increase in load losses, also because the empty spaces between the catalyst granules are small.
Catalysts having a shape other than the conventional ones are described in U.S. Pat. No. 4,441,990, which relates to tubular extruded granules that have an essentially triangular or quadrangular multilobed cross-section. These catalysts provided advantages in terms of resistance to breakage and pressure drop, but the results are not really very much different from those that can be obtained with conventional catalysts.
Extrusion is the process adopted industrially to form catalysts.
This process is technologically very simple to perform; however, it has the drawback that it is absolutely unsuitable for forming complex shapes that can offer a satisfactory solution to the above mentioned problems.